Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Vehicle maker Oshkosh (NYSE: OSK) popped 13% in intraday trading today on news that activist investor Carl Icahn is angling for some time in the driver's seat.

So what: An SEC filing revealed that Icahn and his affiliated funds purchased a 9.5% stake in Oshkosh between April 29 and June 30. The filing also stated that Icahn considers the shares undervalued and hopes to have discussions with management about ways to increase shareholder value -- e.g., the stock's return.

Now what: Even after today's pop, the stock trades at a mere five times trailing EPS and 9.9 times forward EPS and is down about 9% year to date. Earnings expectations are low due to concerns about the outlook for spending in two sectors that account for most of the company's revenue: defense and state and local government. In addition to Icahn's involvement, potential industry consolidation and/or Oshkosh's construction and specialty truck business could give more power to the stock.

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Fool contributor Cindy Johnson owns shares of OSK. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.